1. Field of the Invention
This invention relates to the use of thiolcarbamates as herbicides and miticides. More particularly, this invention relates to a process for preparing a high purity thiolcarbamate in high yield by reacting an amine, a carbonylsulfide and an alkyl halide which process avoids pollution problems in industrial drainage.
2. Description of the Prior Art
It is known to prepare thiolcarbamates by a variety of processes including:
1. A secondary amine, carbon monoxide and sulfur are charged into a reactor vessel, and reacted at high temperature and pressure to give an amine salt of the corresponding thiolcarbamic acid. The amine salt of the thiolcarbamic acid thus obtained is then reacted with an alkyl halide to give the thiolcarbamate as shown in the following reaction scheme (U.S. Pat. No. 3,151,119) ##EQU4## However, because of the low stability of the thiolcarbamic acid amine salt formed in the process as an intermediate, as well as the fact that this process necessarily involves the use of heat and pressure, significant amounts of decomposition products and/or by-products are formed which results in low yields of low purity thiolcarbamate product. Moreover, the most serious drawback with this process is that, as mentioned above, it generates appreciable amounts of undesired products which cause serious ecological problems. Because of the pollution problems posed by this process, it is impractical industrially.
2. In another process, carbonylsulfide is reacted with a secondary amine in the presence of an alkali hydroxide to give an alkali metal salt of thiolcarbamic acid which is further reacted with an alkyl halide in acetone, methanol or ethanol to give a thiolcarbamate as shown by the following reaction scheme (Japanese Patent Publication No. 28427/1973). ##EQU5## However, the alkali metal salt of thiolcarbamic acid produced in the first step of this process is obtained as an aqueous solution, while the alkyl halide is not soluble in the water system. Accordingly, in order to perform the reaction in a homogeneous system, it is necessary to use a hydrophilic organic solvent such as acetone, methanol or ethanol, which is very undesireable because the hydrophilic organic solvent used unavoidably contaminates the industrial drainage and thereby pollutes rivers. Thus, this process again is not applicable as a practical industrial process because of the ecological problems.
A need, therefore, continues to exist for a method of producing thiolcarbamates which is industrial practical and which overcomes the heretofore longstanding pollution problems.